Because of their high strength and toughness, various duplex alloys have been used in the field of iron alloys known as “special steels.” A number of special steels with various two-phase structures have been in practical use. A representative special steel is the duplex stainless steel standardized under Japanese Industrial Standard JIS SUS329J1.
In the ceramics field, on the other hand, research and development of single-phase ceramics and putting them to practical use is being promoted. By the use of combustion synthesis under controlled pressure and temperature, we have succeeded in synthesizing a “silicon alloy” from inexpensive silicon, which exists in the Earth's crust in large quantities, and are promoting its practical application as a structural material to replace special steels. Duplex ceramics, however, are yet to be developed.
Relevant prior art can be found in unexamined Japanese Patent Publication No. 91272/2004, unexamined Japanese Patent Publication No.162851/2008, Japanese Patent Publication No. 4339352, Japanese Industrial Standard JIS G4304-1999, and in Toshiyuki Watanabe et al., “FC Report 26,” p. 68, Japan Fine Ceramics Association, Tokyo, 2008.
A major difference between fine ceramics and steels as structural materials lies in their plastic deformability. It has been generally recognized that ceramics are brittle, whereas steels are tenacious, and it has been considered that ceramics are unable to replace steels completely because ceramics exhibit brittle fracture morphology.
Because it is a solid solution, a silicon alloy has a good chance to be composed of two phases by utilizing eutectic reaction after combustion synthesis. Ceramics with a duplex structure, combining advantages of each phase, have a good chance to exhibit ductility similar to that of duplex steels, and are expected to be much stronger than traditional single-phase ceramics.
However, only a single-phase silicon alloy has been obtained by conventional combustion synthesis. The reason appears to be that the conventional combustion synthesis process employs a small combustion synthesis apparatus and a cooling apparatus having a high cooling capacity. Therefore the silicon alloy is cooled too rapidly. The same can be said concerning metals such as steels. That is, slow cooling is necessary in order to obtain eutectic properties. Accordingly, there is a good chance that a silicon alloy having a duplex eutectic structure can be obtained by controlling the cooling rate following combustion synthesis.
Such duplex eutectic silicon alloys can be commercialized easily by applying the manufacturing processes described in Japanese Patent Applications Nos. 158407/2009 and 202440/2009, which processes are already in practical use. The latter Japanese patent application corresponds to U.S. patent application Ser. No. 12/871,009, filed Aug. 30, 2010, the disclosure of which is incorporated by reference.
Thus, it can be expected that development of a duplex eutectic silicon alloy will increase the chance for ceramics to replace special steels as general-purpose industrial materials, and will expand the fields in which a silicon alloy metal-ceramics can be utilized. Therefore it is an object of this invention to obtain a eutectic silicon alloy having a duplex structure.